The afterglow of GRB 170817A/GW 170817 was very unusual , slowly rising as F _ { \nu } \propto { } t _ { { } obs } ^ { 0.8 } \nu ^ { -0.6 } , peaking at \sim 150 days , and sharply decaying as \sim { } t _ { { } obs } ^ { -2.2 } .
VLBI observations revealed an unresolved radio afterglow image whose flux centroid apparently moved superluminally with v _ { app } \approx 4 c between 75 and 230 days , clearly indicating that the afterglow was dominated by a relativistic jet ’ s compact core .
Different jet angular structures successfully explained the afterglow lightcurves : Gaussian and steep power-law profiles with narrow core angles \theta _ { c } \lesssim 5 ^ { \circ } and significantly larger viewing angles \theta _ { { } obs } / \theta _ { c } \sim 3 - 5 .
However , a top-hat jet ( conical with sharp edges at \theta = \theta _ { 0 } ) was unanimously ruled out since it appeared to produce an early flux rise much steeper ( \propto { } t _ { { } obs } ^ { a } with a \gtrsim 3 ) than observed .
Here we clearly show that 2D relativistic hydrodynamic simulations of an initially top-hat jet can fit GW 170817/GRB 170817A ’ s afterglow lightcurves and flux centroid motion , for \theta _ { { } obs } / \theta _ { 0 } \approx 3 and \Gamma _ { 0 } = \Gamma ( t _ { 0 } ) \gtrsim 10 ^ { 2.5 } where t _ { 0 } is the simulation ’ s start time , and find a minimal jet energy of E _ { { } min } \approx 2 \times 10 ^ { 48 } erg .
On the dynamical time a bow-shock like structure develops with slower material on the jet ’ s sides , whose emission is not strongly beamed away from observers at \theta _ { { } obs } \gtrsim 2 \theta _ { 0 } , causing a shallow flux rise .
The steep initial flux rise is an artifact caused by the simulation ’ s finite t _ { 0 } , missing its flux contributions from t < t _ { 0 } and sometimes “ compensated ” using an analytic top-hat jet .
While an initial top-hat jet is not very physical , it can still reproduce the afterglow data , which require that most of the jet ’ s energy initially resides in a narrow core and sharply drops outside of it .